Sandvik Springflex™ SH spring wire

Disclaimer: Recommendations are for guidance only, and the suitability of a material for a specific application can be confirmed only when we know the actual service conditions. Continuous development may necessitate changes in technical data without notice. This datasheet is only valid for Sandvik materials.

Tensile strength can be increased by 200 MPa (29 ksi) up to 450 MPa (65 ksi) by tempering depending on tensile and tempering conditions. Please see heat treatment for further information. The tensile strength variation between spools/coils within the same production lot is ±50 MPa (7 ksi) maximum. Yield strength in the tempered condition is approx. 90 % of the tempered tensile strength. Tensile strength values are guaranteed and are measured directly after production. During storage, both the tensile strength and yield strength will increase somewhat due to ageing. Depending on the storage conditions, the tensile strength can increase by 0 - 80 MPa (0 - 12 ksi).

The tensile strength and yield strength will decrease by 3–4% per 100°C (184oF) increase in service temperature.

Straightened lengths

Straightening will reduce the tensile strength by approx. 7 %.

Fatigue strength

The diagrams below are based on tempered and pre-stressed cylindrical helical springs with a wire surface free from tooling damage.

At elevated temperatures, the fatigue strength decreases at100°C (210°F): by about 5 %200°C (390°F): by about 10 %

Relaxation and setting limit

Diagrams are based on tests with tempered springs.

Figure 4. Setting limit and maximum permissible shear stress. Setting limit curve A, and maximum permissible shear stress, curve B, as a function of the wire diameter. The setting limit is defined as the shear stress at which the relaxation is 2% after a load time of 24 hours. Curve B lies 25% below curve A.

Figure 5. Relaxation 24 hours. Relaxation (load loss) at various shear stresses as a function of service temperature. This diagram refers to a wire diameter of 1.0 mm (0.039 in.).

Shear modulus, MPa (ksi)

Modulus of elasticity, MPa (ksi)

Corrosion resistance

Sandvik Springflex™ SH is a process development of Sandvik Springflex™ . The chemical composition is identical for both grades and, therefore, the corrosion properties detailed below are also valid for both grades.

General corrosion

The most common environments, where general corrosion occurs in stainless steels, are strongly acidic or alkaline solutions. The specific composition of the environment is crucial for corrosiveness, and may change drastically if oxidizing or reducing compounds are added. The performance of stainless steel grades can vary considerably in the same environment and to different additives. It is, therefore, extremely important that the environment, where a product is to be used, is characterized thoroughly. When this is done, a suitable material can usually be selected. The economic advantages of choosing a grade with high corrosion resistance, sometimes acquired at a higher price per kilo, can be illustrated by estimating life cycle cost. In most media, Sandvik Springflex™ SH possesses better resistance to general corrosion than steel of type ASTM 316L.

Pitting

The pitting resistance of a steel is determined primarily by its chromium and molybdenum contents, but also by its nitrogen content, as well as its slag composition and slag content. A parameter for comparing the resistance of different steels to pitting is the PRE number (Pitting Resistance Equivalent).

The PRE is defined as, in weight-%:PRE = % Cr + 3.3 x % Mo + 16 x % NThe PRE numbers for Sandvik Springflex™ SH and two standard materials are given in the following table.

Grade

% Cr

% Mo

% N

PRE

Springflex SH

22

3.2

0.18

>35

ASTM 316L

17

2.2

-

24

ASTM 302/304

18

-

-

18

The ranking given by the PRE number has been confirmed in laboratory tests. This ranking can generally be used to predict the performance of an alloy in chloride containing environments.

Laboratory determinations of critical temperatures for the initiation of pitting (CPT) at different chloride contents are shown in the figure below. The chosen testing conditions have yielded results that agree closely with practical experience.

Sandvik Springflex™ SH can be used at considerably higher temperatures and chloride contents than ASTM 302/304 and ASTM 316 without pitting. It is, therefore, far more serviceable in chloride-bearing environments than standard austenitic steels.

Crevice corrosion

Crevice corrosion is in principle the same as pitting corrosion, but occurs in crevices and cracks, e.g. between flange joints, under deposits on the metal surface or in welds with incomplete penetration. Crevice corrosion often occurs at lower temperatures and at lower chloride contents than those necessary for pitting to occur. Resistance is influenced by the content of Cr, Mo and N, in the same way as pitting resistance.

Stress corrosion cracking

Duplex stainless steels are far less prone to this type of corrosion. Laboratory tests have shown the good resistance to stress corrosion cracking of Sandvik Springflex™ SH. Results from these tests are presented in the diagrams below. The first diagram indicates the temperature-chloride range within which Sandvik Springflex SH and the standard steels ASTM 302/304 and ASTM 316L can be used without risk of stress corrosion cracking.

Results of laboratory tests carried out in calcium chloride are shown in the next diagram. The tests have been continued to failure or a max. test time of 500 h. The diagram shows that Sandvik Springflex™ SH has a much higher resistance to SCC than the standard austenitic steels ASTM 302/304 and ASTM 316. In aqueous solutions containing hydrogen sulphide and chlorides, stress corrosion cracking can also occur in stainless steels at temperatures below 60°C (140°F). The corrosivity of such solutions is affected by acidity and chloride content.

Laboratory tests of Sandvik Springflex™ SH have confirmed the good resistance to stress corrosion cracking in environments containing hydrogen sulphide. This has also been verified by available operating experience.

Heat treatment

Spring tempering will increase the tensile strength by 200 MPa (29 ksi) up to to 450 MPa (65 ksi) depending on tensile and tempering conditions. We recommend 450°C (840°F)/1–3 h for batch tempering. To obtain best results, when tempering in a continuous conveyor furnace, where holding time at full temperature is very short, the temperature should be increased preferably to about 500°C (930°F). The holding time should be 3 - 10 minutes.

Shorter times may result in uneven tempering.

Bending

The minimum bending radius should not be less than half the wire diameter. The wire surface should be free from any tooling damage because slight imperfections in the surface can lead to fracture, even at large bending radii.

Disclaimer: Recommendations are for guidance only, and the suitability of a material for a specific application can be confirmed only when we know the actual service conditions. Continuous development may necessitate changes in technical data without notice. This datasheet is only valid for Sandvik materials.